Silver halide materials which are used to generate halftone dot images, also called screened images, for use in the photolithographic printing process are commonly referred to as "lith" films. An essential characteristic of lith film is that it has an extremely high contrast; generally greater than 10. There are two commercial processes which are used to achieve this high contrast effect. The first process, referred to herein as "conventional lith", utilises a silver halide emulsion of high chloride content which is processed in a developing solution containing hydroquinone, as the principle or sole developing agent, and a low quantity of sulphite ion. The second process, referred to herein as the "hydrazine-lith" process, involves the incorporation of a hydrazine or hydrazide derivative in a negative-acting, surface latent image silver halide emulsion and processing in a developer of high pH (generally greater than 11). The hydrazine derivative may alternatively be present in the developer solution but this is not preferred.
The hydrazine-lith process is disclosed in U.S. patent specification Nos. 2,419,975, 4,168,977 and 4,224,401. Modifications and improvements to the process are disclosed in U.S. patent specification Nos. 2,419,974, 2,410,690, 4,269,929, 4,166,742, 4,221,857, 4,237,214, 4,241,164, 4,311,871, 4,243,739 and 4,272,614. In all of the foregoing patents, where a preference is expressed as to the grain size distribution of the silver halide emulsion, it is stated that monodisperse or narrow grain size distribution emulsions are most suitable. A monodispersed emulsion is defined as one in which 90% of the silver halide grains have a grain size in the range of .+-.40% of the average grain size.
U.S. patent specification No. 4,444,865 describes a method by which the covering power of core shell type direct positive emulsions may be increased by combining two silver halide emulsions of different grain size. The method disclosed relates to direct positive emulsions of the internal latent image type which contain a hydrazine or other nucleating agent. This direct positive process is quite different in mechanism, method of manufacture and application from the negative-acting hydrazine-lith process.
Japanese Patent Application No. 57-58137 discloses a silver halide light-sensitive photographic material which contains a compound represented by the following general formula: EQU R.sup.1 --NHNH--COR.sup.2 (I)
in which:
R.sup.1 is an aryl radical which may or may not be substituted, and PA1 R.sup.2 represents a hydrogen atom, an aryl radical which may or may not be substituted, or an alkyl radical which may or may not be substituted, PA1 R.sup.3 represents an aromatic or aliphatic group, PA1 R.sup.4 represents a hydrogen atom or an alkyl, aryl, alkoxy or aryloxy group, any of which groups may be substituted, and PA1 G represents a carbonyl, sulphonyl, sulphoxy, phosphoryl or an N-substituted or un-substituted amino group. PA1 each R.sup.5 may be the same or different and is selected from substituents such that the total Hammet sigma value of the R substituents is less than 0.3, and PA1 R.sup.6 represents a hydrogen atom, an optionally substituted phenyl group or an optionally substituted alkyl group containing 1 to 10 carbon atoms.
in an emulsion layer and/or some other hydrophilic colloid layer, forming a silver halide emulsion exhibiting a grain size distribution curve possessing at least one peak in the fraction with grain size below 0.4 micron and at least one peak in the fraction with grain size above 0.7 micron, but without a peak in the fraction exceeding grain size of 0.4 micron but not reaching grain size of 0.7 micron, the light sensitive photographic material possessing at least one silver halide emulsion layer on a support. It is stated that if the fine grain emulsion exceeds 0.4 micron there is a reduction in Dmax and if the coarse grain emulsion is below 0.7 micron the sensitivity is extremely low and is considered to be unsuitable for practical application.
It has now been found, contrary to the teaching of the prior art, that hydrazine-lith type silver halide emulsions may comprise a combination of two emulsions having a grain size below 0.4 microns and provide suitable sensitivity, contrast, and density for graphic arts, lith, line and scanner applications.